Kinugawa Onsen, known as the "okuyashiki," or "living room," of Tokyo, was opened to development in the early Meiji period, and it became a major destination for those wanting to escape the noise and congestion of the city. The name "Kinugawa" literally means Angry Demon River. The exact provenance is unclear, but the most likely explanation is that this comes from the raging waters within — although the river is now dammed and considerably more placid.
The central area is home to several hotels and ryokan, most with their own hot springs. Unfortunately, due to a decline in group tours and the failure of the area's regional bank, Ashikaga Bank, in the late 1990s, several of the larger hotels have gone under, and their empty hulks scar an otherwise picturesque location. Still, the resort is home to dozens of hotels, pensions, and ryokan, and the area--together with nearby Kawaji--still attracts over 2 million visitors each year.
If you can spare the cash and time, it may be worth it head up northward to Kawaji or one of the many tiny hot spring hamlets collectively known as Oku-Kinu (奥鬼怒, "Inner Kinu").
Tobu runs all-reserved limited express services, known as Tokkyū (特急) trains, to the area. These trains, which use Tobu's "SPACIA" railroad equipment, have comfortable, reclining seats, with vending machines available on most trains.
The Kinu (きぬ) limited express departs from Asakusa every 30-60 minutes, and reaches Kinugawa-Onsen (鬼怒川温泉) in 2 hours at a cost of ¥2800. Ordinary rapid trains are cheaper at ¥1500, but take anywhere from 2 hours 20 minutes to 3 hours to reach Kinugawa. Kawaji is another 20 minutes up the line.
The Kinugawa Theme Park Pass  includes a round-trip fare and access to one or both of Kinugawa's most famous theme parks: Nikko Edo Mura and Tobu World Square. Valid for 2 days. Cost ¥3300-6000, depending on the theme park chosen and the adult/child fare. This pass is available only to those with a non-Japanese passport.
By JR and Tobu
The trains, called the Kinugawa and Spacia Kinugawa, depart from Shinjuku station at 10:35, 13:05 and 17:35. Return service departs Kinugawa-Onsen at 8:13, 10:36 and 15:03.
In additon, a limited express train departs from Shinjuku at 7:12 for Nikko. You can transfer from this train at Shimo-Imaichi (下今市) for a shuttle train service to Kinugawa. The last service to Shinjuku departs Kinugawa-Onsen at 16:24 (Shuttle train connecting at Shimo-Imaichi to the limited express).
Seat reservations are mandatory, and the one-way fare between Shinjuku and Kinugawa-Onsen is ¥3900. If you plan to use this train in both directions, you should purchase a JR Tobu Nikko Kinugawa Free Pass for ¥7800, which includes one round-trip on the limited express and unlimited usage of local Tobu trains and buses in both the Kinugawa and Nikko areas within a three day period.
The new limited express service is fully covered under the JR East Rail Pass; national Japan Rail Pass holders can use it for ¥1560 each way (covering the portion of the trip between Kurihashi and Kinugawa-Onsen). The Japan Rail Pass does not cover Tobu trains or buses, and the JR East Rail Pass only covers local Tobu trains between Shimo-Imaichi and Tobu Nikko, and Shimo-Imaichi and Kinugawa-Onsen. You will have to pay separate fares for any services that are not covered.
You can alternatively take the Shinkansen from Tokyo Station to Utsunomiya, change to the JR Nikko line for Imaichi, and then change again to the Tobu line for the final leg, but this is unlikely to be worth the hassle even if you have the Japan Rail Pass.
Kinugawa is fairly spread out. You can either use the infrequent buses, or the expensive taxis. If arriving by train, be sure to check if your lodgings are closer to Kinugawa Onsen or Kinugawa Kōen station.
There is little to see in Kinugawa Onsen itself, but the Nichien Momiji Line, the highway connecting Kinugawa and Kawaji, makes for a fairly scenic drive.
Three theme parks in the area, collectively known as Kinugawa Theme Park, are major draws for Japanese visitors:
Loll about in hot springs. More adventurous types may also want to try battling against angry demons by rafting in the Kinugawa River.
Most guests eat at their lodgings, but there are a scattering of restaurants just outside Kinugawa Onsen station.
The recession of the 1990s hit Kinugawa hard and many hotels struggle with low occupancy rates (or have been outright shut down). This means there are some pretty good bargains to be found, especially off-season.